Adding Si to crystallize primary Si and eutectic Si is effective for improvement of abrasion resistance and stiffness of an aluminum alloy. By increasing an amount of addition of Si, an amount of crystallization increases and these properties are improved. However, the amount of addition has a limitation since the liquidus temperature increases as the amount of addition increases. Given this, in a case in which further improvement in the properties is required, other crystallized products such as an Al—Fe—Si compound, an Al—Ni compound, an Al—Ni—Cu compound and the like must be used. In order to obtain these crystallized products, Fe, Ni and Cu are added. Among these additive elements, Ni and Cu may lead to increased cost of an aluminum alloy, while Fe is low in cost. However, the Al—Fe—Si compound coarsens as the amount of crystallization increases, leading to deterioration of mechanical properties such as strength, extension, fatigue and the like, and consequently lowered processability.
Generally, Mn or Cr is added in order to avoid coarsening of the Al—Fe—Si compound in the aluminum alloy. However, in a case in which a large amount of Fe is added, a sufficient refining effect cannot be obtained.
As a refinement means in a case of large amount of addition of Fe, for example in Patent Document 1, with respect to 1 to 4% by mass of Fe, a content of Si is adjusted to be 1.7×Fe content+13 to 13.7% by mass; a content of Ti is adjusted to be 0.05 to 0.07×Fe content+0.1% by mass; a content of Cr is adjusted to be 0.1×Fe content+0.05 to 0.15% by mass; and a content of Mn is adjusted to be 0.4 to 0.6×Fe content, and ultrasound is emitted above the liquidus temperature.
By emitting ultrasound toward molten aluminum alloy above the liquidus temperature, the number of embryos, which form the basis for crystal nuclei in molten aluminum, increases. This generates a large number of crystal nuclei and allows crystallization of fine crystallized products. In addition, by adjusting components and composition ranges of the molten aluminum alloy as described above, the crystallization products are crystallized in a short period of time, in an order of: the Al—Ti compound; the Al—Cr compound; the Al—Fe—Si compound; and Si. As a result, the Al—Ti compound and the Al—Cr compound are made to act as nuclei of the Al—Fe—Si compound.
In addition, the present inventors have proposed, in Patent Document 2, adding silicide particles having high temperature stability which act as solidification nuclei of the Al—Fe—Si compound. As the silicide, CrSi2, TiSi2, WSi2, MoSi2, ZrSi2, TaSi2, NbSi2, and the like can be assumed. Melting points of the abovementioned metal silicide are 1500 to 2000° C. Even if a melting point is 1500 to 2000° C., the silicide held in molten metal dissolves at some point; however, with the high melting point, the silicide can be present as a solid phase for a while and can act as a solidification nucleus.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2010-090429
Patent Document 2: PCT/JP2012/075692